There is an optical disk apparatus in which a light beam emitted from a laser is focused on a rotating disk to reproduce signals recorded in the disk.
FIG. 25 is an enlarged view of a part of a typical compact disk (hereinafter referred to as CD). In the figure, reference numeral 1 designates a base substrate of the CD, numeral 2 designates an information surface, and numeral 3 designates a series of pits on the information surface. These pits 3 provide a spiral track. The pitch of the spiral track is 1.6 .mu.m. Aluminum is vapor-deposited on the information surface 2 as a reflecting film. The rear surface of the disk, opposite the information surface 2, is irradiated with a light beam. When information recorded on the track is reproduced, focusing is performed so that a focal point of the light beam is always positioned on the information surface 2 where the Al reflecting film is present, and tracking is performed so that the focal point of the light beam is positioned on the track. The diameter of the disk is about 120 mm, and the thickness of the base substrate 1 is 1.2 mm. The wavelength of the light beam is 780 nm.
In recent years, disks with high recording density, for example, a digital video disk (hereinafter referred to as DVD) in which digital image data are recorded, have been proposed. Hereinafter, a DVD will be described as an example of a high recording density disk. However, the present invention is not restricted to a DVD.
The recording density of the DVD is about five times as high as that of the CD. In order to achieve this recording density, the track pitch is reduced to 0.74 .mu.m and the track recording density (number of data on the track per a unit length) is increased. With the increase in the recording density, the wavelength of the light beam is reduced to 650 nm. In order to reliably reproduce information recorded in the disk even when the disk is inclined, the thickness of a base substrate of the disk is 0.6 mm, that is, thinner than that of the CD. The diameter of the DVD is approximately equal to that of the CD. Hereinafter, this DVD is called a single-layer DVD.
Besides the single-layer DVD mentioned above, there is also a double-layer DVD as shown in FIG. 26. In FIG. 26, reference numeral 4 designates a first layer, numeral 5 designates a second layer, numeral 6 designates an intermediate layer, and numeral 10 designates a base substrate of the DVD. The DVD is irradiated with a light beam at the rear surface of the base substrate 10. Like the single-layer DVD, the first layer 4 wherein information is recorded is 0.6 mm distant from the rear surface of the base substrate 10. A reflecting film comprising Au or the like is employed as the first layer 4, and the reflectivity is reduced to about 35%. So, a part of the light beam passes through the first layer 4. This first layer serves as a first information surface. The second layer 5 is located on the first layer 4 with the 40 .mu.m thick intermediate layer 6 between them. The light beam passing through the first layer 4 is reflected by the second layer 5 and travels through the intermediate layer 6, the first layer 4, and the base substrate 10. This second layer serves as a second information surface. Thereby, information recorded in the first layer 4 and the second layer 5 can be reproduced. The second layer 5 comprises aluminum or the like, and the reflectivity is about 90%. The recording capacity of this double-layer DVD is about twice as large as that of the single-layer DVD. In this double-layer DVD, when information recorded in the first information surface 4 is reproduced, focusing is performed on the first information surface 4. When information recorded in the second information surface 5 is reproduced, after the focal point is moved from the first information surface 4 to the second information surface 5, focusing is performed on the second information surface 5.
When a CD and a DVD are reproduced using an optical disk apparatus, the apparatus is provided with an optical system for reproducing the CD and an optical system for reproducing the DVD because the CD and the DVD have different base material thicknesses, and the optical systems are switched according to a disk set in the apparatus. In order to identify whether the disk set in the apparatus is a CD or a DVD, a cartridge containing the disk has a hole for discrimination.
However, when the disk is not contained in a cartridge, it is impossible to identify the disk using a cartridge. If the optical system for the DVD is used for the CD, information is not correctly reproduced because of the difference in base material thicknesses between the CD and the DVD.
Meanwhile, there is a disk called CD-R which is a recordable optical disk of write once read many type. When the CD-R, which employs the same optical system for the CD, is set in the optical disk apparatus, if the optical system for the DVD is selected by mistake, information recorded in the CD-R is sometimes destroyed. This destruction is attributed to the fact that the CD-R usually employs, for a recording film, an organic coloring material that highly absorbs light having a wavelength of 650 nm and, therefore, the recording film is easily destroyed.